A-V blocks, encountered frequently in cardiac patients, arise when electrical impulses flowing from the SA node along the conduction bundles are delayed when they reach the A-V junction/A-V node. In some pathologies, if an A-V delay is sufficiently great, the ventricles will beat at their own intrinsic and slower rate. With A-V blocks in other pathologies, the ventricles can beat at a variable and/or intermittent rate, or ectopic foci can appear, potentially leading to life threatening ventricular fibrillation.
A variety of strategies have been employed for pacemakers to overcome the adverse physiological effects of A-V blocks. One such strategy is overdriving or overpacing, in which the pacemaker stimulates the ventricles at a faster rate than the atrial beating rate. A problem encountered with such strategies is that the atrial and ventricular beating can not be coordinated for optimal pumping efficiency. Another problem is that such fast ventricular pacing rates fatigue the heart because physiological and biochemical functioning generally are not optimized. Furthermore, such additional fatigue only imposes greater restraints on the already limited life style of the typical cardiac patient. Thus, the patient with an already weakened heart can be subjected to unnecessary overstimulation, and be stressed and further weakened as a result of application of current pacemaking protocols.
Patented technologies relating to overdriving pacing with subsequent relaxation of the pacing rate include U.S. Pat. No. 5,626,620 to Kieval. et al., which discloses a pacemaker stimulation protocol in which fusion and/or near fusion beats are detected by monitoring changes in the characteristics of the evoked QRS. The protocol is adjustable to allow selection of an acceptable percentage of fusion beats. When an unacceptable fusion percentage is measured, the A-V delay is automatically decreased to lead to a higher ventricular beating rate from the pacemaker's synchronous pace pulses (ventricular "capture"). Once ventricular capture is maintained for a predetermined time interval or number of cycles without an unacceptable rate of fusion, the A-V interval is incrementally increased to produce a beating rate toward the rate at which fusion had previously occurred. Upon again meeting an unacceptable fusion percentage, the A-V delay is automatically decreased, and the cycle continues so as to approximate the longest A-V interval (i.e., the slowest ventricular beating rate) consistent with avoiding fusion.
U.S. Pat. No. 5,527,347 to Shelton, et al. discloses a pacemaker ventricular stimulation protocol in which the A-V delay is slowly increased until fusion occurs, at which point the A-V delay is decreased slightly. The cycle is then repeated. Thus, the A-V delay is cyclically maintained in a small range of about that corresponding to fusion, to slightly lower values (i.e., higher ventricular beating rate).
U.S. Pat. No. 5,522,858 to van der Veen discloses a pacemaker stimulation protocol in which A-V delays are gradually decreased until ventricular tracking of atrial firing occurs. In particular, the ventricles are stimulated after the atrial depolarization impulse reaches the ventricles, but are not stimulated during the ventricular refractory period. The net effect is to decrease the prolonged A-V delay period, and thus increase the ventricular beating rate. In small increments, the A-V delay period then is further decreased until ventricular tracking is observed.
U.S. Pat. No. 5,480,413 to Greenhut, et al. discloses a means for using a pacemaker to correct ventricular beating rate instability in the presence of atrial fibrillation/tachyarrhythmia. First, ventricular beating is decoupled from atrial beating by gradually increasing the ventricular beating rate (dual or multichamber pacemakers are switched to a single chamber pacing mode) via appropriately spaced electrical stimulations. Once a stabilized beating rate is achieved at the higher ventricular beating rate, then the rate of ventricular stimulation is slowly decreased to the lowest rate that provides ventricular rate stability, and held at this rate until the atrial tachyarrhythmia/fibrillation disappears. Dual or multi-chamber (atria and ventricles) pacemaking is then resumed.
U.S. Pat. No. 5,441,522 to Schuller discloses a dual chamber pacemaker stimulation protocol in which the A-V interval is cycled between two values when retrograde conduction from ventricular stimulation renders the atria refractory to the normally timed stimulation by the pacemaker. When such a condition is sensed, the A-V interval is shortened to one value. Once a predetermined time or number of pulses has occurred, or once a spontaneous ventricular reaction is sensed within the shortened A-V interval, then the longer A-V interval is restored.
U.S. Pat. No. 5,340,361 to Sholder discloses a ventricular stimulation protocol in which the A-V interval is automatically adjusted to just less than that for the intrinsic (and pathological) rhythm to produce a ventricular firing that is slightly in advance of the intrinsic ventricular firing time. This invention overcomes the problem of abnormal A-V delay, which decreases cardiac efficiency due to non-optimal atrial-ventricular synchronization. The rates of atrial firing and ventricular firing are equal in this invention.
U.S. Pat. No. 5,334,220 to Sholder discloses a ventricular stimulation protocol in which the A-V interval is automatically adjusted to avoid ventricular stimulation at a time that would result in fusion (at the cross-over point) with the endogenous ventricular stimulation. A final A-V value is selected by incrementally adjusting the A-V interval until the crossover point is reached with respect to the R wave. The final A-V value that is set is based on the determined cross-over point, adjusted by a small margin. Thus, this procedure overdrives the intrinsic rhythm to ensure a suitably short A-V interval/delay that, otherwise, would impair cardiac pumping efficiency. When this procedure is invoked (automatically) too frequently, it is suspended for a predetermined period.
U.S. Pat. No. 5,105,810 to Collins, et al. discloses a cyclic protocol for achieving the minimum voltage for ventricular pacing for the purpose of extending the life of batteries used in pacemakers. The protocol uses a series of bradyeardia support pacing pulses at a predetermined voltage, and ventricular pressure measurements are analyzed during the pulse train to determine if capture has occurred. If capture has occurred during the pulse train, bradycardia support pacing pulses again are delivered once the stimulus voltage has been decreased by a step. If capture is the result, then the decremental voltage stepping and capture assessing is continued until capture is lost, at which point the voltage is incrementally increased until capture occurs.
U.S. Pat. No. 4,503,857 to Boute, et al. discloses a ventricular pacing protocol in which either spontaneous bradycardia or tachycardia is altered first by ventricular capture, followed by gradual increase or decrease, respectively, in the rate of pulse pacing until a normal programmed pacing rate is reached.
As can be seen from earlier inventions, pacemakers utilize overdrive ventricular pacing that adjusts the A-V interval/delay in a manner that avoids fusion, and that controls ventricular firing solely by the imposed pacing impulses. However, such protocols have not been optimally designed to minimize the energy expenditure of the already compromised patient's heart. Generally, the above references are designed to change the stimulation rate by adjustment of the A-V interval/delay in order to achieve a predetermined rate or a physiological standard.
What is needed is a pacemaker with a ventricular firing protocol that minimizes the energy of the heart used for contraction/pumping work. Furthermore, what is needed is a pacemaker with a ventricular firing protocol in which the maximum overdrive pacing rate is only slightly (i.e., only a few beats per minute--ideally two or three beats per minute) greater than the atrial firing rate at the commencement of the first cycle of the protocol. In addition, what is needed is a pacemaker for ventricular firing that uses a pacing protocol that achieves re-synchronization/fusion, so as to produce the least amount of stress on a heart which may already be in a weakened condition.
Lastly, an improved means for stimulating muscle tissue, wherein the contraction elicited is enhanced and the damage to the tissue adjacent to the electrode is diminished, is also desired. Enhanced myocardial function is obtained through the biphasic pacing of the present invention. The combination of cathodal with anodal pulses of either a stimulating or conditioning nature, preserves the improved conduction and contractility of anodal pacing while eliminating the drawback of increased stimulation threshold. The result is a depolarization wave of increased propagation speed. This increased propagation speed results in superior cardiac contraction leading to an improvement in blood flow. Improved stimulation at a lower voltage level also results in reduction in power consumption and increased life for pacemaker batteries.